fix(rpm-versionlock): warn when dnf5 lockfile unreadable on Python <3.11 (#1462)

[AquaticAzelf]

Fixes #1462

get_dnf5_locks() returned [],[],[] when tomllib is unavailable (Python <3.11) even if /etc/dnf/versionlock.toml exists, silently reporting OK.

• check-plugins/rpm-versionlock/rpm-versionlock:568 now checks for the dnf5 lockfile existence when tomllib is None and returns [],[],["<lockfile> cannot be read, this needs Python 3.11 or newer"] via the ignored channel, which main() already treats as STATE_WARN and displays under "These configurations are read but applied to nothing".
• A host without dnf5 (no lockfile) stays quiet.
• check-plugins/rpm-versionlock/unit-test/run removes the NEEDS_TOMLLIB skip and makes those 7 testcases assert WARN with cannot be read, this needs Python 3.11 or newer when tomllib is missing, so the same tests pass on both old and new interpreters.

Verified folder-locally: python ./run passes (48 tests) on Python 3.14 and simulated tomllib=None correctly returns WARN for existing lockfile and OK for missing file.

[markuslf]

Thanks for the PR. We have already fixed #1462 in be274bd